In recent years attention has been paid to high-speed communication systems, such as WiMAX (Worldwide Interoperability for Microwave Access), which cover wide coverage areas. In communication systems such as WiMAX, communication is usually performed between a base station connected to an upper network by a wired link and a mobile station. In some cases, however, a relay station for performing relay transfer by radio is installed between a base station and a mobile station. This makes it possible to extend a communication area or improve the communication throughput of a mobile station.
At present IEEE Std 802.16-2009 (2009-05-29), for example, is standardized by IEEE (Institute of Electrical and Electronics Engineers) as a radio interface in a mobile WiMAX system. Furthermore, IEEE Std 802.16j-2009 (2009-06-12), for example, is standardized as extended specifications of its relay communication function. In addition, in order to accommodate IMT-Advanced which is a fourth generation mobile telecommunication system and which is in the process of being standardized by ITU-R (International Telecommunication Union—Radio communication sector), standardization of more advanced specifications of a radio interface is in progress and IEEE Draft P802.16m/D2 (2009-10-14), for example, is framed as a draft.
By the way, when a base station transmits data to a mobile station via a relay station, a time lag occurs in the relay station. As a result, there is a case where the mobile station cannot receive the data at timing at which the mobile station is to receive the data.
Accordingly, a radio relay communication method by which receiving or transmission timing is controlled with time which a radio relay station takes to perform a relay process (occurrence of a time lag) taken into consideration for the purpose of making it possible for a mobile station or a radio base station to correctly receive a message or data at scheduled timing was proposed in the past (see, for example, Japanese Laid-open Patent Publication No. 2008-48218).
With Japanese Laid-open Patent Publication No. 2008-48218), however, in order to make it possible for the mobile station to correctly receive data at scheduled timing, the base station manages transmission timing in which a time lag that occurs in the relay station is taken into consideration. This increases the load on the base station.
FIG. 21 is a view for describing an example of a communication system in which a time lag that occurs in a relay station is taken into consideration. FIG. 21 indicates the sequence of the operation of a base station, a relay station, and a mobile station.
A period 101 indicated in FIG. 21 is a period in which a mobile station receives data. A period 102 is a period in which a base station transmits the data. The periods 101 and 102 are equal in timing with a case where the mobile station is under the control of the base station taken into consideration.
A period 103 is a period in which the base station transmits the data to the mobile station via the relay station. The period 103 is earlier than the periods 101 and 102 with a time lag D101 which occurs in the relay station taken into consideration. As a result, the mobile station under the control of the relay station can correctly receive the data transmitted in the period 103.
The base station manages in this way not only the period 102 but also the period 103 in which the time lag D101 that occurs in the relay station is taken into consideration. This increases the load on the base station.